As geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers. Techniques of this type, known generally as optical metrology, operate by focusing an optical beam on a sample and then analyzing the reflected energy. Ellipsometry and reflectometry are two examples of commonly used optical techniques. For the specific case of ellipsometry, changes in the polarization state of the optical beam are analyzed. Reflectometry is similar, except that changes in magnitude of the reflected intensities are analyzed. Scatterometry is a related technique that is used when the structural geometry of a subject creates diffraction (optical scattering) of the incoming probe beam. Scatterometry systems analyze diffraction to deduce details of the structures that cause the diffraction to occur.
Techniques of this type may be used to analyze a wide range of attributes. This includes film properties such as thickness, crystallinity, composition and refractive index. Typically, measurements of this type are made using reflectometry or ellipsometry as described more fully in U.S. Pat. No. 5,910,842 and 5,798,837 both of which are incorporated in this document by reference. Critical dimensions (CD) including line spacing, line width, wall depth, and wall profiles are another type of attributes that may be analyzed. Measurements of this type may be obtained using monochromatic scatterometry as described in U.S. Pat. Nos. 4,710,642 and 5;164,790 (McNeil). Another approach is to use broadband light to perform multiple wavelength spectroscopic reflectometry measurements. Examples of this approach are found in U.S. Pat. No. 5,607,800 (Ziger); U.S. Pat. No. 5,867,276 (McNeil) and U.S. Pat. No. 5,963,329 (Conrad) (each of the patents is incorporated in this document by reference). Still other tools utilize spectroscopic ellipsometric measurement. Examples of such tools can be found in U.S. Pat. No. 5,739,909 (Blayo) and U.S. Pat. No. 6,483,580 (Xu). Each of these patents and publications are incorporated herein by reference.
Photo-modulated reflectance (PMR) is another technique used to perform nondestructive inspection and analysis of semi-conductor wafers. As described in U.S. Pat. No. 4,679,949 (incorporated in this document by reference), PMR-type systems use a combination of two separate optical beams. The first of these, referred to as the pump beam is created by switching a laser on and off. The pump beam is projected against the surface of a subject causing localized heating of the subject. As the pump laser is switched, the localized heating (and subsequent cooling) creates a train of thermal waves in the subject. The second optical beam, referred to as the probe beam is directed at a portion of the subject that is illuminated by the pump laser. The thermal waves within the subject alter the reflectivity of the subject and, in turn, the intensity of the reflected probe beam. A detector synchronously samples the reflected probe beam synchronously with the switching frequency of the pump laser. The resulting output is used to evaluate parameters such as film thickness and material composition.
In wafer fabrication environments, optical metrology systems inspect wafers at various stages during the production cycle. To avoid contaminating the wafers, these systems typically include electromechanical wafer handling systems. The handling system includes a robot arm for transferring wafers from a cassette into the measurement region. Various motion stage combinations are used to move wafers with respect to the measurement or probe beam. For example, stages with X and Y linear travel coupled with a theta stage for rotating the wafer are common. Other combinations include polar coordinate stage systems that rotate the wafer and move the wafer along only a single linear axis.
Optical metrology systems typically follow a measurement recipe where the wafer is successively positioned so that very specific sites are selectively aligned with the probe beam. As the feature size on the semiconductor wafers continue to shrink, very accurate positioning of the wafer is necessary to insure proper measurement. Unfortunately, the stage motion systems typically do not have the precision to permit accurate positioning merely by instructing the stage where to move. Rather, in a typical operation, instructions are given for a particular set of stage movements to bring the probe beam near the measurement site. At this point, the lens of an imaging system is moved into position and an image of the wafer is recorded. Pattern recognition software is used to determine the current wafer location and whether a corrective move is necessary. The imaging lens is then removed and the optics for focusing the probe beam are moved in position. The selected measurement can then be made. Typically, this process is divided into two stages. In the first stage, a measurement recipe is created using the following steps:                1) Using the wafer imaging system and wafer positioning controls, the operator identifies each measurement site on the wafer.        2) For each measurement site, the operator uses a computer graphical user interface and imaging system to identify a vision model. The vision model is a portion of the wafer image that will be used to identify the associated measurement site.        3) The displacement from the measurement site to the vision model is recorded.        4) The identity of the die within the wafer that includes the measurement site is recorded.        
In the second stage, the measurement recipe is used as part of the production process.                To use the measurement recipe, the following steps are used:        1) The metrology tool positions the wafer by selecting the die and the expected position of the measurement site within the die. At that location, the metrology tool acquires an image of the wafer.        2) The metrology tool searches for the vision model associated with this measurement site in the acquired image.        3) If the metrology tool finds the vision model, it calculates an offset between the location of the vision model and the expected location of he vision model (i.e., the position in which the vision model was located during the development of the measurement recipe).        4) The metrology tool uses the offset along with the displacement from the vision model to the measurement site to determine the actual location of the measurement site.        5) The metrology tool performs a corrective move and positions the wafer for measurement.        6) The tool performs the measurement.        
In practice, the process of searching and identifying the vision model is often time consuming. It may also require positioning of the mechanical parts of the measurement system (such as lenses, etc.) to enable the acquisition of the image by the wafer imaging system. As a result, the use of imaging tends to have an undesirable impact on the rate at which the metrology tool performs the measurement process.
One approach to dealing with this problem is to very accurately characterize the movement of the stage in advance. This can be done by issuing a series of movement commands to the stage system. At each location, the actual positions of the stage and the expected positions (stage error) are compared and stored. Over time, a complete correction map of the stage performance can be generated. Thereafter, during measurements, when a move command is to be generated, the map can be consulted and suitable offsets added to the command so that the wafer will be moved to the correct position. Measurement without a prior visual inspection of location can be considered “blind positioning.”
While the above approach could produce the desired result, in practice it is difficult to implement. This follows because it is generally necessary to complete a very large number of test movements of differing lengths and differing directions to construct an adequate map. The large number of movements means that construction of complete stage maps can be very time consuming. Complete maps also fail to compensate for wear induced changes in stage movement. As a result, complete maps become increasingly inaccurate over time.
Based on the preceding, it is clear that there is a need for better methods for positioning wafers in optical metrology systems. This is particularly true for production environments where wafer throughput must constantly be increased.